The present invention relates to microwave devices and, more particularly, to a sliding load for calibrating a microwave network analyzer. A major objective of the present invention is a sliding load calibrator providing more accurate calibration of a microwave network analyzer.
Microwave network analyzers are used to determine certain performance parameters for a wide variety of passive microwave devices. A typical network analyzer includes an output port and an input port, each of which includes a coaxial transmission line connector. A "device under test" (DUT) is coupled between the input port and the output port using coaxial cables. The network analyzer transmits broadband signals through the DUT to determine its power transmission and reflection characteristics, commonly referred to as "S" parameters.
Inevitably, the network analyzer itself causes some of the signal attenuation and reflection that it measures. The contributions of the network analyzer to the parameters it is measuring need to be known so that its contribution can be mathematically compensated to yield an accurate characterization of the DUT.
The network analyzer can be characterized by calibrating it using devices of known signal characteristics in place of a DUT. Three standard devices are the short, the open and the sliding load. Many network analyzers are programmed to calibrate themselves on the basis of measurements obtained when these standard devices are in place. The short and the open are simple devices, not much bigger than the network analyzer connector to which they are attached. Accordingly, there has not been much difficulty manufacturing devices which satisfactorily approximate ideal shorts and opens.
There has been considerable difficulty in approximating an ideal sliding load with a commercially practicable device. Typically, a sliding load is an elongated device defining a coaxial air line with a connector at one end and a microwave attenuator at the other. The connector is used to engage a complementary connector of a network analyzer. The microwave attenuator can be moved toward and away from the connector to control the length of the air line. The effect of this movement is to obtain parameter measurements over a range of phases for each frequency in the broadband transmission of the network analyzer. In theory, by measuring the parameters of interest over at least 180.degree. of phase changes, the error components due to the network analyzer can be mathematically isolated from those due to the sliding load. However, this analysis is impaired to the extent that the sliding load differs from the ideal.
One significant deviation from the ideal sliding load occurs at the connection between the sliding load and the network analyzer. An ideal connection would be electrically indistinguishable from a continuous airline. To achieve this, it would be necessary for the connecting center conductors to butt against each other when the outer conductors are butted against each other. This would require perfect axial alignment of center and outer conductor for each of the mating connectors.
An attempt to manufacture connectors with axially aligned center and outer conductors would result in some connectors with the center conductor projecting slightly beyond the respective outer conductor and in some connectors with the center conductor set back with respect to the outer conductor. In the event that two connectors with projecting center conductors are mated, a defective airline would result significantly impairing microwave transmissions. The defect would result from the distortion in the center conductors as they push against each other under the torque used to thread together connectors until their outer conductors join.
By manufacturing coaxial microwave connectors with a small nominal center conductor setback, the percentage of connectors with projected center conductors can be greatly diminished, and the odds of mating two connectors with projecting center conductors can be made statistically insignificant. The price of this manufacturing conservatism is a gap between the mated center conductors. The center conductors are electrically connected with a pin from the male conductor extending across the gap and received within a hole of the mating female connector. However, since the center conductors are not butted, the pin defines abrupt narrowings of the conductor comprising the two mated center conductors. These dimensional variations in center conductor diameter define a center conductor gap which perturbs a signal traversing the connection.
The disturbance caused by such a connector gap is related to its length in such a way that the contributions of each connector to the disturbance is related to their respective setbacks. When a network analyzer is used to characterize a DUT, the raw measurements reflect the setback of the network analyzer connector as well as of the mating connector. Thus effects of the network analyzer setback must be known to permit the DUT to be effectively evaluated.
Due to their function, calibration devices are often manufactured to greater precision than standard connectors. Simple calibration devices such as shorts and opens are manufactured with their relatively short center conductors aligned with the respective outer conductor reference plane. Sliding loads are considerably longer than shorts and opens, and for that reason alone it is difficult to achieve the same degree of conductor alignment.
Furthermore, to provide for a variable-length airline, the center conductor must extend unsupported for a considerable distance within the outer conductor. The relatively thin and unsupported center conductor can be displaced radially relative to the outer conductor without much force. Whereas, alignment of the shorter and therefore better supported center conductors of opens and shorts is ensured by proper alignment of their outer conductors, proper mating of a sliding load center conductor requires special procedures.
Typically, these special procedures require that the sliding load center conductor be free-floating, i.e., be movable axially relative to the outer conductor. Given this condition, the center conductor can be projected in front of the respective reference. So projected, it can be visually aligned with a mating center conductor of a network analyzer connector. Once the center conductors are engaged, the outer conductors can be threaded together until butted together at the reference plane. As the outer conductors are moved together, the sliding load center conductor retracts relative to the respective outer conductor, but remains butted against the connector center conductor. The end result is that the sliding load center conductor extends past the reference plane to eliminate the gap that would otherwise be present due to setback of the network analyzer connector.
Obscured by the projected center conductor, the effects of the network analyzer connector setback fail to be accounted for in the calibration calculations. Therefore, the effects of the network analyzer setback are erroneously attributed to a DUT during device testing. Thus, in providing for proper center conductor alignment during connection, the accuracy of the calibration procedure is compromised.
What is needed is a sliding load which more closely approaches an ideal sliding load. In particular, a sliding load is needed which can be connected to a network analyzer with the contact surface of the center conductor aligned with the reference plane. Yet provision must still be made for visual alignment of center conductors prior to the butting of outer conductors.